Field of the Invention
The present invention relates to a linear encoder.
Description of the Related Art
It is known that a linear encoder conventionally includes a plate-type scale and a head which moves along the longitudinal direction of the scale. The scale has graduations formed on a surface thereof. The graduations are arranged along the longitudinal direction of the scale. The head has a reading part which detects an amount of a relative movement of the head with respect to the scale by reading the graduations formed on the scale.
For example, a scale apparatus (a linear encoder) described in JP 04-102423 Y includes a scale and a detection head. The detection head includes a movable member such as a bearing, which is situated so as to abut against a surface of the scale. With this configuration, the detection head is able to move smoothly along the longitudinal direction of the scale.
The movable member also functions as a spacer to maintain a distance between the scale and the detection head.
For a linear scale, as a method by which a reading part of a head reads graduations formed on a scale, an electromagnetic induction method, a capacitance method, a photoelectric method, and the like are applicable. Whatever method is used, a distance between the graduations formed on the scale and the reading part situated on the head is desirably small, because reducing the above distance improves the accuracy of detecting an amount of a relative movement of the head with respect to the scale. In the scale apparatus described in JP 04-102423 Y, an electromagnetic induction method is applied as a method by which the detection head reads the graduations formed on the scale.
FIG. 6 is a diagram illustrating a conventional linear encoder. FIG. 7 is a lateral view of the conventional linear encoder. More specifically, FIG. 6 illustrates the linear encoder as viewed from the scale side, and FIG. 7 illustrates the linear encoder of FIG. 6 as viewed from the upper side of FIG. 6.
As shown in FIGS. 6 and 7, the conventional linear encoder 100 includes a plate-type scale 110, and a head 120 configured to move along the longitudinal direction of the scale 110 (the right-left direction in FIGS. 6 and 7).
The scale 110 has graduations 111 formed on a surface thereof (a surface on the head 120 side). The graduations 111 are arranged along the longitudinal direction of the scale 110. The graduations 111 are formed of a pattern of annular-shaped scale coils 111A regularly arrayed at predetermined pitch along the longitudinal direction of the scale 110. The scale 110 is made of glass. Also, the scale coils 111A are formed in rows of three as counted in the transverse direction of the scale 110.
The head 120 has a reading part 121 configured to detect an amount of a relative movement of the head 120 with respect to the scale 110 by reading the graduations 111 formed on the scale 110. The reading part 121 includes an excitation coil 121A formed in a substantially annular shape, and a plurality of detection coils 121B formed inside the excitation coil 121A. The excitation coil 121A is formed in a substantially elliptic shape having a long axis extending along the longitudinal direction of the scale 110. Further, the excitation coil 121A enclosing the detection coils 121B are formed in rows of three as counted in the transverse direction of the scale 110.
In the above linear encoder 100, when an electric current is applied to the excitation coil 121A, an electromotive current is generated in the scale coil 111A forming the graduations 111, and subsequently in the detection coil 121B. Then, when the head 120 moves with respect to the scale 110, electromagnetic couplings among the coils 111A, 121A, and 121B change depending on an amount of a movement of the head 120. Based on this, the linear encoder 100 detects a sinusoidal signal with a period equal to the pitch of the scale coils 111A, through the detection coil 121B. By reference to the sinusoidal signal, the linear encoder 100 detects an amount of a relative movement of the head 120 with respect to the scale 110. In other words, the linear encoder 100 is configured as a linear encoder of electromagnetic induction type.
FIG. 8 is an exploded perspective view of the conventional linear encoder, showing a scale and a head in separated positions.
The head 120 includes, as shown in FIGS. 6 to 8, three bearings 122 each having a rotation axis extending along the transverse direction of the scale 110, and two bearings 123 each having a rotation axis extending along a direction in which the scale 110 and the head 120 face each other. The head 120 is placed in such a manner that a peripheral surface of the bearing 122 is pressed against a surface of the scale 110, and at the same time a peripheral surface of the bearing 123 is pressed against a lateral surface of the scale 110.
As shown in FIG. 7, each of the bearings 122 is situated on the head 120 so as to project above the reading part 121 toward the scale 110. This means the bearing 122 functions as a spacer which maintains a distance between the scale 110 and the head 120 by abutting against the surface of the scale 110.
With this configuration, when the head 120 moves with respect to the scale 110, the bearing 122 rolls on the surface of the scale 110. In the linear encoder 100, therefore, as shown in FIG. 6, a path of the bearing 122 is spaced apart from the graduations 111. This prevents the bearing 122 from making contact with the graduations 111 when rolling. If the bearing 122 touches the graduations 111, the bearing 122 may damage the graduations 111, or a distance between the graduations 111 and the reading part 121 changes due to an uneven surface caused by the graduations 111, which decreases the measurement accuracy of the linear encoder 100.